1. Field of the Invention
The present invention relates to a pixel circuit used in an Active Matrix Organic Light Emitting Diode display (referred to as “AMOLED Display” hereinafter) and the like, a driving method thereof, and a display device which is provided with the pixel circuit. While an organic light emitting diode is also referred to as an organic EL element, it is referred hereinafter as “OLED (Organic Light Emitting Diode)”.
2. Description of the Related Art
There is no standard pixel circuit of AMOLED display, so that each of the companies manufacturing AMOLED display uses their original design pixel circuits. Hereinafter, a basic pixel circuit will be described. FIG. 11A is a circuit diagram showing the basic pixel circuit, FIG. 11B is a waveform chart showing a driving method thereof, and FIG. 11C is a graph showing the output characteristic of a driving TFT (Thin Film Transistor) included in the pixel circuit.
A pixel circuit 900 includes a switch TFT 901, a driving TFT 902, a capacitor 903, and an OLED 904, and it is driven and controlled by a double transistor system. The switch TFT 901 and the driving TFT 902 are both p-channel type FET (Field Effect Transistor). The gate terminal of the switch TFT 901 is connected to a scanning line 905, and the drain terminal of the switch TFT 901 is connected to a data line 906. The gate terminal of the driving TFT 902 is connected to the source terminal of the switch TFT 901, the source terminal of the driving TFT 902 is connected to a power supply line 907 (power supply voltage VDD), and the drain terminal of the driving TFT 902 is connected to the anode terminal of the OLED 904. Further, the capacitor 903 is connected between the gate terminal and the source terminal of the driving TFT 902. A power supply line 908 (power supply voltage VSS) is connected to the cathode terminals of the OLED 904.
When a selection pulse (scan signal Scan) is outputted to the scanning line 905 and the switch TFT 901 is set on with this structure, a data signal Vdata supplied via the data line 906 is written to the capacitor 903 as a voltage value. The retention voltage written to the capacitor 903 is held through one frame period, the conductance of the driving TFT 902 is changed in an analog manner by the retention voltage, and a forward bias current corresponding to a luminous gradation is supplied to the OLED 904.
Through driving the OLED 904 by a constant current in this manner, the light emission luminance of the OLED 904 can be maintained to be constant even when the resistance value of the OLED 904 changes due to deterioration.
In order to compensate variation and fluctuation in the threshold voltage of the transistor that drives the OLED in such type of pixel circuit, there is known a technique for detecting the threshold voltage (see U.S. Unexamined Patent Application Publication 2013/0169611 (Patent Document 1) and Japanese Unexamined Patent Publication 2012-128386 (Patent Document 2), for example). As the threshold voltage detecting technique, following two types are the mainstream. (1) A technique (diode connection type) with which the gate terminal and the drain terminal are connected and an electric current is flown between the drain terminal and the source terminal to automatically bring the gate-source voltage Vgs to be close to the threshold voltage Vth. (2) A technique (source follower type) with which the potential of the gate terminal is fixed and an electric current is flown between the drain terminal and the source terminal to automatically bring the gate-source voltage Vgs to be close to the threshold voltage Vth. The source follower type is advantageous in respect that the threshold voltage Vth can be detected even in a depression type transistor in which an electric current flows even when Vgs=0 V.
However, there are following issues with the existing pixel circuit having the threshold voltage detecting function.
(1) The threshold voltage detection period is limited to one horizontal scanning period, so that the compensation accuracy of the threshold voltage becomes deteriorated when the display resolution becomes higher.
Detection of the threshold voltage is executed in the time where a reference voltage is supplied from a data line within one horizontal scanning period or in the time where a data voltage is supplied from a data line within one horizontal scanning period (see FIG. 4 of Patent Document 1, FIG. 4 of Patent Document 2, for example). Thus, though it is possible to detect the threshold voltage for more than one horizontal scanning period, crosstalk is generated due to an influence of the data voltage to be supplied to the neighboring pixel circuits. Therefore, the period to detect the threshold voltage must be shorter than one horizontal scanning period.
In the meantime, the more the display resolution increases, the shorter one horizontal scanning period becomes. When one horizontal scanning period becomes shorter, the threshold voltage detection period becomes shorter as well. Thus, before the gate-source voltage Vgs reaches the threshold voltage Vth, it is required to complete detection of the threshold voltage. Thereby, detection accuracy of the threshold voltage is deteriorated, so that compensation accuracy of the threshold voltage is worsened as well.
(2) Due to the hysteresis characteristic of the driving transistor, several frames are required to completely change the black image to the white image even though the image data has been already completely re-written from black to while. This phenomenon is generally called image retention. In other words, if an electric current is not flown to the driving transistor for a long time, the hysteresis characteristic of the driving transistor becomes initialized. Thus, even when a white-display Vgs bias determined based on the initialized hysteresis characteristic is applied, the electric current is instantly decreased by the hysteresis characteristic for lighting up so that it is insufficient for providing the original brightness of white display.
In consideration of the above-mentioned circumstances, it is an object of the present invention to achieve a pixel circuit to improve the accuracy for detecting the threshold voltage. In addition, another object of the present invention is to achieve a pixel circuit to avoid image retention.